Alkali metal titanates have electrochemical properties which make them desirable as electrode materials for a variety of devices. Lithium titanate (Li4Ti5O12 or LTO) has been found to have particular utility as an electrode material for lithium batteries. It is a relatively low-cost material, and exhibits high performance characteristics in lithium batteries; consequently, it is anticipated to have significant utility as an electrode material for high performance, high power batteries such as those utilized in hybrid electric vehicles and other high power applications.
One important characteristic of high power, high performance batteries is rate capability. That is, the rate at which the batteries can take up and deliver an electrical charge. This parameter is particularly important under high charge/discharge rates as are encountered in electric vehicles and other high power applications.
First cycle reversibility is another very important parameter for rechargeable lithium batteries. This parameter measures the decline in storage capacity when a freshly manufactured lithium battery is initially cycled. Manufacturers compensate for this initial loss by building extra capacity into batteries. However, this approach increases the size and cost of batteries, and industry has always sought to limit magnitude of first cycle reversibility.
Various lithium titanate materials are commercially available and are utilized in the manufacture of lithium batteries. However, heretofore available commercial materials produce lithium batteries having first cycle reversibilities of approximately 80%, which represents a significant inefficiency. In addition, prior art batteries incorporating lithium titanate materials suffer from problems of high rate charge and discharge capacities, and these limitations are a significant detriment to their use in high power applications for electric vehicles, power tools and the like. In an effort to improve high rate performance, the prior art has believed that lithium titanate spinel materials having extremely high surface areas and correspondingly small particle sizes, typically in the nanoscale regime below 100 nanometers, must be employed in order to obtain good high rate performances. Such materials are disclosed, for example, in U.S. Pat. No. 7,211,350. Conventional wisdom in the prior art is that high rate capability cannot be expected from larger particle size and correspondingly lower specific surface area materials. Processes for the preparation of high surface area, nanoscale lithium titanate materials as well as processes and structures for their incorporation into battery systems are complex, expensive and difficult to implement. Therefore, it will be appreciated that there is a need in the art for improved lithium titanate electrode materials which can provide good performance characteristics in terms of high rate capacities and improved first cycle reversibilities in batteries incorporating such materials. In addition, such materials and batteries should be simple and economical to fabricate and utilize. As will be explained in detail hereinbelow, the present invention provides improved lithium titanate materials which manifest very good performance characteristics even when implemented in particle size formats outside of the prior art nanoscale range. These and other advantages of the invention will be apparent from the drawings, discussion and description which follow.